A gas turbine engine includes a number of rotor sections axially aligned, each having a hub (or portion of a common hub) with a plurality of equally spaced rotor blades mounted on the hub. A shroud encompasses the blade tips with as little clearance as possible in order to minimize bypass flow of air or other gases past the tips of the blades. The shroud is substantially but not necessarily exactly coaxial, because it is very difficult to fabricate and maintain a shroud that is exactly round and located right at the blade tips, particularly with some flexing of the shroud.
One solution is to utilize a clearance sealing layer on the shroud that is abraded by the blade tips, thus producing a self-adjusting, relatively tight seal, for example as disclosed in U.S. Pat. No. 4,540,336 (Cawley). In the higher temperature sections of an engine a ceramic type abradable material is necessary such as described in U.S. Pat. No. 4,280,975 (Ammann). However, shroud materials, particularly ceramics, have a tendency to wear the tips of the blades which generally are formed of a metal, changing the dimensions and configurations of the blade tips designed for the engine. In the case of titanium blades, metallic friction against the shroud is a concern for fire.
Coatings have been provided on rotor blade tips to alleviate these problems. One example is thermal sprayed chromium oxide on a titanium blade. Another is low pressure plasma sprayed nickel cobalt-chromium-aluminum-yttrium alloy with abrasive SiC grit imbedded therein, on a nickel superalloy blade tip. Yet another is boron nitride in a metal matrix, brazed to the tip. However a need still exists for an improved ceramic material for rotor blade tips, having lower friction and combined with higher abrasive qualities, and lower cost.
One convenient method of applying coatings is thermal spraying. Thermal spraying, also known as flame spraying, involves the heat softening of a heat fusible material such as metal or ceramic, and propelling the softened material in particulate form against a surface which is to be coated. The heated particles strike the surface where they are quenched and bonded thereto. Conventional thermal spray guns are used for the purpose of both heating and propelling the particles. In some types of thermal spray guns, the heat fusible material is supplied to the gun in powder form. Such powders are typically comprised of small particles, e.g., between 100 mesh U.S. Standard screen size (149 microns) and about 5 microns.
High velocity thermal spraying such as with a plasma gun such as in U.S. Pat. No. 3,145,287 (Siebein et al) produces relatively dense coatings. Another type of thermal spraying involves a high velocity oxy-fuel (HVOF) gun, such as taught in U.S. Pat. No. 4,865,252 (Rotolico) and in U.S. Pat. No. 4,416,421 (Browning). In HVOF, oxygen and fuel are supplied at high pressure into a combustion chamber such that the flame issues from a nozzle at supersonic velocity. In either plasma or HVOF powder fed into the flame is heated and propelled at high velocity to produce a dense coating.
A number of ceramic materials are utilized in the thermal spray process, for example zirconia plasma sprayed onto blades for thermal barrier or corrosion protection, as taught in U.S. Pat. No. 4,576,874. Metallic bond coats are often used as further taught in this patent. Aluminum oxide is a conventional thermal spray material. U.S. Pat. No. 4,588,655 (Kushner), and a paper "Some Recent Developments of Flame- and Plasma-Spraying Powders" by H. R. Eschnauer and B. Krismer, International Thermal Spray Conference, Miami Fla. (September 1976), describe the thermal spraying of alloyed zirconium oxide and aluminum oxide; no particular applications are disclosed in these references.
An object of the present invention is to provide an improved rotor blade for a gas turbine engine having a plurality of rotor blades and a substantially coaxial shroud encompassing the tips of the blades. Further objects are to provide such a blade having a tip layer of ceramic, to provide such a blade having improved friction characteristics against shroud material, to provide such a blade having improved ability to abrade shroud material, to provide such a blade having improved resistance to tip wear, and to provide an improved method for producing such a rotor blade.